This invention relates to a process for recovering pure maleic acid from aqueous solutions containing benzoquinone especially those catalytic benzene oxidation processes.
The technical production of maleic anhydride is generally carried out by catalytic vapor phase oxidation of benzene or C.sub.4 -carbon atoms, as butene, butadiene and butane. The reaction gases occurring thereby are cooled after leaving the reactor such that maleic anhydride is condensed.
However, the separation temperature for the water in the reaction gases, must be kept considerably below the dew point, otherwise maleic anhydride is hydrated to maleic acid. At temperatures necessary for separation, mostly below 50.degree. C, the vapor pressure of the maleic anhydride is such that considerable quantities of maleic acid remain in the waste gas.
To reduce losses these waste gases are then washed with water. A solution containing maleic acid results, called acid water, which can be concentrated up to 30 to 50 wt.% maleic acid. Maleic anhydride can be recovered from these acid waters for example by means of an organic solvent, mostly a technical xylene mixture. The acid waters are first dewatered and then the maleic acid is dehydrated to maleic anhydride.
At larger throughput, the dehydration is carried out in a continuously working dehydration column. Here the acid water is led to an xylene vapor stream in the column, whereby the evaporation and the dehydration heat are taken from the xylene stream under simultaneous use of the xylene as carrier stream for the azeotropic removal of water.
The column is filled for this purpose with Raschig rings or with suitable plates. The product from the sump of the dehydration column (maleic anhydride and xylene) is separated by distillation in a distillation column and afterwards the raw maleic anhydride is reworked in a further distillation to pure maleic anhydride.
The most important raw material for recovering maleic anhydride is benzene.
It is known that in continuously working dehydration columns, as far as they are fed with acid waters of benzene oxidation plants, black, tar-like precipitations occur which hamper the correct working of the column.
For avoiding these disadvantages, it was suggested to use distillation columns with column plates in which openings with movable valve plates adjusted in a cage are placed and whereby the openings are rounded to a venturi profile.
This means higher capital costs, for a dehydration column for avoiding of corrosion must be made at least partly of solid material.
This invention provides a chemical process for preventing the formation of such precipitations in the dehydration column. Examining the precipitations formed, it was found that the formation of water-insoluble precipitations can be prevented if a small quantity of benzoquinone is present in the acid water from the benzene oxidation plant. Benzoquinone occurs as by-product from the benzene oxidation. According to former experiences, the formation of benzoquinone cannot be wholly prevented. Benzoquinone occurs mainly in the acid water while in the maleic anhydride melt a small amount also appears. The acid waters often show benzoquinone quantities of 200 to 500 mg/l while higher contents are not uncommon.
Other impurities of the acid waters, like aldehydes, which usually exist in considerably higher concentrations (for example, 5 to 10 g/l acid water) do not form water insoluble precipitations in the dehydration column.